Isotopic evaluation on relative contributions of major NOx sources to nitrate of PM2.5 in Beijing

Song, Wei; Wang, Yan-Li; Yang, Wen; Sun, Xin-Chao; Tong, Yin-Dong; Wang, Xue-Mei; Liu, Cong-Qiang; Bai, Zhi-Peng; Liu, Xue-Yan

Isotopic evaluation on relative contributions of major NOx sources to nitrate of PM2.5 in Beijing

2019

Nitrate (NO₃⁻) is a key component of secondary inorganic aerosols and PM₂.₅. However, the contributions of nitrogen oxides (NOₓ) emission sources to NO₃⁻ in PM₂.₅ remain poorly constrained. This study measured nitrogen (N) isotopes of NO₃⁻ (hereafter as δ¹⁵N-NO₃⁻) in PM₂.₅ collected at Beijing in 2014. We observed that δ¹⁵N-NO₃⁻ values in PM₂.₅ (−2.3‰ − 19.7‰; 7.3 ± 5.4‰ annually) were significantly higher in winter (11.9 ± 4.4‰) than in summer (2.2 ± 2.5‰). The δ¹⁵N differences between source NOₓ and NO₃⁻ in PM₂.₅ (hereafter as Δ values) were estimated by a computation module as 7.8 ± 2.2‰ − 10.4 ± 1.6‰ (8.8 ± 2.4‰). Using the Δ values and δ¹⁵N values of NOₓ from major fossil (coal combustion, vehicle exhausts) and non-fossil sources (biomass burning, microbial N cycle), contributions of major NOₓ sources to NO₃⁻ in PM₂.₅ were further estimated by the SIAR model. We found that seasonal variations of δ¹⁵N-NO₃⁻ values in PM₂.₅ of Beijing were mainly caused by those of NOₓ contributions from coal combustion (38 ± 10% in winter, 20 ± 9% in summer). Annually, NOₓ from coal combustion, vehicle exhausts, biomass burning, and microbial N cycle contributed 28 ± 12%, 29 ± 17%, 27 ± 15%, and 16 ± 7% to NO₃⁻ in PM₂.₅, respectively, showing actually comparable contributions between non-fossil NOₓ (43 ± 16%) and fossil NOₓ (57 ± 21%). These results are useful for planning the reduction of NOₓ emissions in city environments and for elucidating relationships between regional NOₓ emissions and atmospheric NO₃⁻ pollution or deposition.

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